Image transducers



Sept. 8, 1959 w. o. REED IMAGE TRANSDUCERS Filed Jan. 18, 195e @SRS NW.

WILLIAM O. REED INVENTOR.

Hls ATTORNEY.

1| En: Ex

"Maig-Alga United States Patent IMAGE TRANSDUCERS William 0. Reed, Wellesley, Mass., assignor to The Rauland Corporation, a corporation of Illinois Application January 18, 1956, Serial No. 559,852

1` Claim. (Cl. Z50-213) This invention relates to image transducers such as image converters and the like. Many devices known in the electrical and optical arts produce an output'image which is a replica of an input image. For example, in pickup and storage tubes it is known tov utilize a photosurface of restricted wavelength response to emit electrons only upon the incidence of energy of the proper Wavelength. The electrons thusl emitted may be directed by any known means to impinge upon a fluorescent screen having a different wavelength response characteristic, thereby illuminating the screen and producing an output image which is an exact replica of the input image but which is translated in frequency, as from the invisible to the visible spectrum. It is difficult to realize either high sensitivity or very high gain from such an elementary system, and various unsuccessful attempts have been made to increase the amplification. Moreover, it is difficult to improve the sensitivity of such a device because it depends in large measure upon the photocathode material used and the quantity of light energy which energizes the photosurface.

It is an object of this invention to produce both an accurate and a highly sensitive image transducer.

It is a further object of this invention to produce such a transducer which is responsive to two light sources, a first source which conditions the transducer for operation and a second source which activates the transducer to produce an output image.

It is a further object of this invention to produce a transducer which is sensitive to radiation in a Wavelength region (i.e., the infra-red of wavelength longer than 1.2 microns) to which the presently known photoemissive surfaces, such as SbCs3 or Ag-O-Cs, are not responsive.

In accordance with the invention, an image transducer includes a fluorescent screen and a photocathode, which is affixed to a surface substantially parallel to the fluorescent screen and is transparent to infra-red radiation, for emitting electrons in response to the incidence of visible light energy. A perforated target electrode comprising a conductive support element is disposed substantially parallel to and intermediate the fluorescent screen and the photocathode; the target electrode further comprises a layer of material facing the photocathode which has a characteristic impedance that is a function of the intensity of incident infra-red radiation. interposed between the photocathode and the target electrode is means for accelerating the electrons to impinge on the target electrode at less than first cross-over velocity and produce thereon a charge suicient to block the passage of electrons to the screen. Means are also included for projecting energy within the infra-red range through the photocathode onto the target electrode to vary the impedance of an image area of the target electrode as determined by such energy, and thus permit electrons to pass through the area to reproduce a corresponding image on the fluorescent screen.

The features of this inveniton which are believed to ICC be novel are set forth with particular-ity in the appended claims. The invention itself, however, together with fur-A ther objects and advantages thereof, may best be understood by reference to the following description taken in,

conjunction with the accompanying drawing, in which the single iigure is a side view, partly in section and partly` tion. .n

The drawing shows a cross-sectional view of an image transducer 10 which is disposed coaxially with an input lens system 11, shown schematically as a convex lens..v

In the illustrated embodiment, transducer 10 comprises a cylindrical portion 12 and a. translucent output windowv 13; both pieces 12 and 13 may be constructed of glass. Two annular electrodes 14 and 15, each of which is a mir4 ror image of the other, are positioned between cylindrical' portion 12 and output Window 13. Each electrode comprises a stem 17 having an outwardly directed ange 16 at one end and an internal base having a feathered sealing edge 18 at the other. Envelope section 12 and window 13 are sealed to projecting edges 18 of electrodes 14 and 15 respectively in any suitable manner as will be well understood in the art. A plurality of U-shaped conductive connectors 19, or a single annular connector of Ulf shaped cross section, mechanically and electrically connect electrodes 14 and 15.

A semi-transparent photocathode 20, sensitive to blue light radiation, is formed on the interior of the input window 35 of transducer 10 opposite output window 13. Photocathode 20 has an S-9 spectral response, 3100-6500 Angstroms (RMA standards) and may be formed, for example, by aiiixing a layer of antimony-cesium (SbCss to the interior of transducer 10. A iirst accelerating grid 21 is disposed in transducer 10 between photocathode 20 and output window 13; grid 21 may be an ordinary mesh grid composed of a number of crossed cylindrical elements supported on their ends, such as is often used for accelerating an electron stream. In accordance with the invention, between grid 21 and output window 13 a perforated target electrode 35, which may include a fine mesh grid 22, is positioned; grid 22 has approximately/1 posited. Film 23 may be formed, for example, by aiiix-` ing a layer of cadmium selenide or a thick layer of antimony trisuliide to grid 22. Of course, suitable precautions must be taken when activating photocathode 20 to avoid caesium contamination or poisoning of infra-red sensitive film 23; suitable techniques for accomplishing this are well known in the art.

A hollow cylindrical accelerating electrode 24 is positioned to be supported by and connected electrically with an inturned iiange of electrode 14. A second accelerating grid 25 is positioned to be supported by and in electrical contact with accelerating electrode 24; grid 25 is placed at the end of electrode 24 opposite the portion contacting electrode 14.

A iiuorescent screen 26 is formed by depositing a suitable luminescent phosphor on the interior portion of output window 13. An aluminum backing film 27 is afxed to iiuorescent screen 26; the compositions of, and techniques for depositing, fluorescent screen 26 and aluminum backing film 27 are well known and understood in the art. Aluminum film 27 is formed to contact a portion of electrode 15.

A source of unidirectional operating potential 28, which in this embodiment is a 10,000 volt direct current power source, is connected in parallel with a potentiometer 29. Photocathode V20 is connected to the negative terminal,

Patented Sept. 8, 1959-- which is also the ground reference, of potentiometer 29. Grid 21 is connected to a 1,000-volt tap on potentiometer 29, and grid 22 is connected to a 3D-volt tap. Electrodes 14 and 15, and therefore grid 25 and aluminum lm 27, are connected to the 10,000 volt terminal of potentiometer 29.

A solenoid-wound focusing coil 30 may be utilized in a known manner to provide a magnetic focusing eld for the electron stream of transducer 10. On the other hand, if the distances between the various electrodes of transducer are short, no additional focusing means is required.

In operation, a source of blue light (not shown) is eX- cited to produce energy represented schematically as ooding light rays 31, which are incident on photocathode as shown in the drawing. Photocathode 20 is constructed to emit electrons upon excitation by energy of a wavelength of the order of blue light. Upon energization of photocathode 20 by the ooding blue light, electrons leave photocathode 20 and are accelerated toward grid 21. After passing through the plane of grid 21, the electron stream is decelerated because grid 22 is 'at a much lower potential than is grid 21. The decelerated electron stream approaches photosensitive film 23 at less than lirst cross-over velocity; i.e., a velocity such that upon impingement the number of secondary electrons emitted by photosensitive ilm 23 is less than the number of incident primary electrons. The incident electron stream charges photosensitive film 23 toward the potential of photocathode 20 and if the impedance of 23 is high enough for the electron current from 20 it may charge slightly below the potential of photocathode 20 by an amount determined by the initial velocity of emission of the photoelectrons and the difference between the contact potentials (or work functions) of film 23 and photocathode 20. The electrons not required to maintain the charge on photosensitive iilm 23 are repelled toward grid 21. Experimentation has shown that photosensitive film 23, if placed within a few thousandths of an inch of photocathode 20, will suiciently accelerate the ooding electrons from photocathode 20 so that grid 21 is not required; however, such a construction introduces structural difliculties, while the particular embodiment illustrated has been found easy to construct and operate.

In accordance with the invention, imaging infra-red light rays 32 from an object exterior to lens system 11 pass through the lens system, through the glass input window 36 of transducer 10, through photocathode 20 and grid 21, and impinge on photosensitive film 23 to form a latent image corresponding to the pattern and 4 intensity of the infra-red rays. The material of photosensitive film 23 is chosen to have a characteristic impedance which varies with the intensity of the incident radiation, within the infra-red range of wavelengths. As the incident radiation strikes lm 23 and lowers its characteristic impedance, photosensitive lm 23 becomes a better conductor and passes electrons from its surface toward the positive potential of grid 22 from whatever area imaging light rays 32 strike. Thus, these areas become more positive than cathode potential by an amount proportional to the intensity of the imaged light rays 32. Electrons emitted from photocathode 20 will now pass grid 22 in those areas in the same proportions and be accelerated toward screen 26.

While a particular embodiment of the invention has been shown and described, it is apparent that modifications and alterations may be made, and it is intended in the appended claim to cover all such modifications and alterations as may fall within the true spirit and scope of the invention.

I claim:

An image transducer comprising: a fluorescent screen; a photocathode, aixed to a surface substantially parallel to said fluorescent screen and transparent to infra-red radiation, for emitting electrons in response to the incidence of visible light energy; a perforated target electrode comprising a conductive support element disposed substantially parallel to and intermediate said fluorescent screen and said photocathode, and further comprising a layer of material facing said photocathode and having a characteristic impedance which is a function of the intensity `of incident infra-red radiation; means interposed between said photocathode and said target electrode for accelerating said electrons to impinge on said target electrode at less than first cross-over velocity and produce thereon a charge sufficient to block passage of electrons to said screen; and means for projecting energy within the infra-red range through said photocathode onto said target electrode to vary the impedance of an image area thereof determined by said energy and permit electrons to pass through said area to reproduce a corresponding image on said fluorescent screen.

References Cited in the file of this patent UNITED STATES PATENTS 2,199,438 Lubszynski May 7, 1940 2,277,246 McGee et al. Mar. 24, 1942 2,322,361 Iams June 22, 1943 2,572,494 Krieger et al. Oct. 23, 1951 2,717,971 Sheldon Sept. 13, 1955 `2,747,131 Sheldon May 22, 1956 

